1. Field of Invention
This invention relates to an electromagnetic flowmeter which uses an exciting current of rectangular waveform supplied to an exciting coil for excitation and, more particularly, to an exciting circuit for such an electromagnetic flowmeter, wherein the duty ratio of current pulses for the ON-OFF control of the exciting current is varied to form a constant exciting current.
2. Discussion of Prior Art
In conventional electromagnetic flowmeters, an alternating voltage at a commercial frequency is applied as it is to an exciting coil, for eliminating the effect of the polarization voltage produced between electrodes, and the resulting magnetic field at the commercial frequency is applied to a fluid being measured. Then the flow rate is detected depending on the alternating voltage produced between the electrodes in contact with the fluid to be measured.
However, there is a limit to the measurement of the flow rate at high accuracy in the electromagnetic flowmeter of this type of excitation system, since induced noises result due to the fluctuation in the magnetic field and vary the zero point.
In view of the above situation, there is also employed an excitation system using an AC current at a lower frequency than the above discussed excitation current applied to the excitation coil for avoiding the effect of the polarization, and converting the form of the excitation current into a rectangular waveform having a period in which the level of the current does not change, and sampling the signal voltage during this period so that the effect of the induced noises may be eliminated.
There are also known various types of excitation systems using lower frequency wave having a rectangular waveform for excitation. One example is described in U.S. Pat. No. 4,462,060.
In the excitation system of U.S. Pat. No. 4,462,060, a commercial AC is applied, after full wave rectification, to a pair of switching elements connected in series with an excitation coil. The switching elements are controlled by a duty cycle converter, including a comparator, that varies the duty cycle corresponding to the value of the excitation current, to maintain the exciting current constant. On the other hand, a gate voltage at a low frequency and having a waveform analogous to that of a desired exciting current is applied to the duty cycle converter to obtain an exciting current of low frequency and having a rectangular waveform whose flat portion is maintained constant.
However, since the level and the waveform of the excitation current are dependent on the level and the waveform of the gate voltage in this type of excitation system, disadvantageously, such level and waveform have to be maintained exactly and control thereof is difficult.
Further, disadvantageously, the voltage obtained through the full wave rectification of the commercial AC voltage is applied as it is to the excitation coil by way of a control circuit for controlling the exciting current. Thus, the system is strictly limited to the power source for which it is designed. For example, the system may be limited to 110 V, and cannot be used for 220 V, in view of the dynamic range of the control circuit.